1. Field of the Invention
Example embodiments of the present invention relate generally to communications systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include one or more base stations or base station routers, which may also be referred to as access points, node-Bs or access networks, for providing wireless connectivity to one or more mobile units, which may also be referred to using terms such as user equipment, subscriber equipment, and access terminals. Examples of mobile units include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop/notebook computers, desktop computers and the like. Each base station may provide wireless connectivity to one or more mobile units, such as the mobile units in a geographical area, or cell, associated with the base station. Alternatively, a base station router may be used to provide wireless connectivity to the mobile units.
Messaging sent from a base station or base station router to one or more mobile units is generally referred to as “forward link” or “downlink” messaging. Messaging sent from a mobile unit to a base station or base station router is generally referred to as “reverse link” or “uplink” messaging.
Orthogonal frequency division multiplexing (OFDM) is an efficient modulation scheme for signal transmission over frequency-selective channels. In OFDM, a wide bandwidth is divided into multiple narrow-band sub-carriers, which are arranged orthogonal with each other. The signals modulated on the sub-carriers are transmitted in parallel.
OFDM may be used to support multiple access for multiple subscribers through time division multiple access (TDMA), in which each subscriber uses all the sub-carriers within its assigned time slots. Orthogonal frequency division multiple access (OFDMA) is another method for multiple access, using the basic format of OFDM. In OFDMA, multiple subscribers simultaneously use different sub-carriers, in a fashion similar to frequency division multiple access (FDMA) (e.g., for each “shared” carrier, frequency divisions are used to allow multiple access).
OFDMA divides a signal into sub-channels (i.e., groups of carriers), with each sub-channel being allocated to a different subscriber. Different sub-channels may then be combined from various carriers. Each subscriber can be treated separately, independent of location, distance from the base station, interference and power requirements. Various modulations can be used for each of the carriers in the system to provide improved coverage and throughput. The sub-channel structure of the OFDMA enhancement enables more efficient duplexing techniques, such as Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), and creates a signal with reduced interference and capable of higher data throughput. In FDD systems, both forward link and reverse link transmissions occur simultaneously on different carriers.
FIG. 1 illustrates a conventional wireless communication system 100. In an example, the wireless communication system 100 may be representative of either an OFDMA system or a Code Division Multiple Access (CDMA) system.
As shown in FIG. 1, the wireless communication system 100 includes a plurality of user equipments (UEs) 105/110 in communication with one or more serving Node Bs 120/125 over an air interface. The plurality of Node Bs are connected to a radio network controller (RNC) 130 with a wired interface. Alternatively, while not shown in FIG. 1, the functionality of both the RNC 130 and Node Bs 120/125 (alternatively referred to as “base stations”) may be collapsed into a single entity referred to as a “base station router”. The RNC 130 accesses an internet 160 through a gateway support node (GSN) 150 and/or accesses a public switched telephone network (PSTN) 170 through a mobile switching center (MSC) 140.
CDMA transmissions have more in-cell interference than OFDMA systems because each CDMA mobile user transmits to a serving base station across an entire frequency band, while OFDMA mobile users are assigned to one of a plurality of sub-bands or sub-channels within the frequency band. Mobile users assigned to different OFDMA sub-channels do not interfere with each other, or at least, only to a negligible degree. However, OFDMA systems may still experience interference because a serving base station typically does not coordinate its assigned OFDMA sub-channels with neighboring base stations. Therefore, a neighboring base station may assign one or more of its OFDMA mobile users to the same sub-channel, causing interference referred to as “outer-cell interference”. Also, a neighboring CDMA system would also cause outer-cell interference because at least a portion of the CDMA transmissions would overlap, in frequency, with the assigned OFDMA sub-channel.
FIG. 2 illustrates a transmitter 200 in a conventional OFDMA system. In an example, the transmitter 200 of FIG. 2 is employed within a transmitting entity (e.g., Node B 120/125, UE 105/110, etc.) within the conventional OFDMA system 100 of FIG. 1.
As shown in FIG. 2, the transmitter 200 includes a modulator 210, a serial-to-parallel (S2P) converter 220, an Inverse Fast Fourier Transformer (IFFT) module 230, a cyclic prefix inserter 240 and a time domain filter 250. The IFFT module 230 includes N ports for receiving modulation symbols. Each of the N ports is associated with an orthogonal sub-carrier. IFFT module 230 is operable to use an N×N IFFT matrix to perform transform operations on its inputs, wherein the entries of the matrix Fj,k are defined as Fj,k=e−2πijk/n, j, k=0,1,2, . . . , n−1 and i=√{square root over (−1)}.
Encoded data symbols are provided as input to modulator 210. Modulator 210 uses well-known modulation techniques, such as BPSK, QPSK, 8PSK, 16QAM and 64QAM, to convert the encoded data symbols into K modulation symbols which are then provided as input to the S2P converter 120, where K≦N. The S2P converter 220 outputs parallel streams of modulation symbols, which are provided as inputs to one or more of the N ports of the IFFT module 230. In the IFFT module 230, an inverse fast Fourier transformation is applied to the modulation symbols to produce a block of chips cn, where n=0, . . . ,N−1. The cyclic prefix inserter 240 copies the last Ncp chips of the block of N chips and prepends them to the block of N chips producing a prepended block. The prepended block is then filtered through time domain filter 250 and subsequently modulated onto a carrier before being transmitted.
FIG. 3 illustrates a mobile user frequency allocation within a conventional OFDMA system. As shown in FIG. 3, users 1, 2, 3, and 4 are assigned to separate orthogonal bandwidth or frequency “chunks” within the total OFDMA frequency band [X . . . Y]. As shown, the total OFDMA frequency band is inclusive of frequencies X and Y, and the frequency band may be established with well-known OFDMA communication protocols. Also shown for each of users 1, 2, 3 and 4 are their associated data rates within each bandwidth chunk.
As shown in FIG. 3, OFDMA base stations attempt to achieve “fairness” by allocating lower data rate mobile users larger chunks of bandwidth. In other words, the areas under the data rate/bandwidth curves associated with the different users 1, 2, 3 and 4 are configured to be roughly equal over time. Thus, OFDMA frequency band allocations to users are typically more efficient, or achieve better total “sector capacity”, when all mobile users transmit at substantially the same data rate, or have the same channel strength, because larger chunks of bandwidth need not be allocated to a lower data rate mobile user at the expense of a higher data rate mobile user.
Additionally, OFDMA comes with the cost of increased signaling and a failure to provide bandwidth sharing for users transmitting at lower data rates and/or requiring fast access to a carrier. In contrast, CDMA systems allow multiple subscriber access without explicit request-and-grant mechanisms, as present in conventional OFDMA, and this may increase user access to the carrier for transmissions.
Certain conventional CDMA systems can approach, and in some cases exceed, the reduced interference characteristic of OFDMA systems. Such a CDMA system may be referred to as Interference Cancellation (IC)-CDMA. IC-CDMA systems are structurally similar to the conventional wireless communication system 100 of FIG. 1. IC-CDMA systems, however, include an interference canceller (IC) at each receiver (e.g., Node Bs 120/125, UEs 105/110, etc.).
A decoding process for mobile users within a conventional IC-CDMA system will now be described. First, a plurality of users 1, 2, 3, etc., are scheduled in a queue for decoding at a receiver with an IC, such as Node B 120/125 within wireless communication system 100 of FIG. 1.
Also, as is characteristic of conventional CDMA systems, each user's transmissions are spread throughout an entirety of the CDMA frequency band, and accordingly interfere with each other. The total IC-CDMA frequency band may span a given range which may be established with well-known IC-CDMA communication protocols.
In conventional IC-CDMA, in order to achieve optimal spectral efficiency, mobile users are decoded one after the other in succession. The IC-CDMA scheduling queue for user decoding is typically arranged from a “best” user to a “worst” user. Thus, a first decoded user is the “best” user, or the user with the highest signal-to-noise ratio (SINR). Likewise, a last decoded user is the “worst” user, and thereby has the lowest SINR.
After the first decoded user is decoded successfully, re-estimation of the channel, re-encoding of the data and cancellation of the first decoded user's decoded signal from the residual received signal is performed. The resulting residual signal is used for decoding the next user (i.e., a second decoded user), and so on. Thus, it will be appreciated that the first decoded user does not receive any benefit of the interference cancellation because it is decoded first, while the last decoded user receives the benefit of interference cancellation from the decoding of each previously decoded user (e.g., the first decoded user, the second decoded user, and so on).
Conventional IC-CDMA systems may be very effective in reducing in-cell interference (e.g., to OFDMA-like levels). Further, unlike OFDMA, the interference cancellation process may also have some beneficial effect upon outer-cell interference. However, because these computations are performed between each user's decoding, the interference cancellation process of IC-CDMA is a computationally expensive procedure. Also, this computational cost scales with the number of users served by a base station within the IC-CDMA system.
OFDMA receivers, by contrast, are less complex than IC-CDMA receivers because they lack the above-described computational requirements of the IC-CDMA interference cancellation process. However, OFDMA is slow to grant mobile users access to sub-channels because the mobile users must request permission to transmit and wait for permission to be granted, by the base station, on a particular sub-channel. Also, OFDMA does not provide bandwidth “sharing” (e.g., superposed transmissions, such as the collective assignment of low data rate mobile users within over-lapping bandwidth chunks), thereby decreasing the spectral efficiency of OFDMA systems. Thus, OFDMA systems can be said to achieve lower interference with relatively simple receivers (e.g., compared to IC-CDMA receivers) at the expense of increased transmission set-up latency and spectral inefficiency.